The present invention relates to electrical connections and formation thereof, and more particularly to electrical connections and formation thereof usable on heat-sensitive substrates or members.
Polyvinylidene fluoride, also referred to as poly(vinylidene fluoride), PVF.sub.2 or PVDF, has received considerable interest as a material for piezoelectric transducers in non destructive testing applications such as acoustic emission and ultrasonic pulse echo testing, particularly for aircraft components under fatigue test. Such transducers are less brittle than ceramic piezoelectric transducers, and have other highly desirable properties. For further information, see Robinson, A. L. Flexible PVF.sub.2 Film: An Exceptional Polymer for Transducers. In Science. 200 (4348): p. 1371-1374. June 23, 1978. Higher frequency transducers are needed to detect smaller critical flaws, but at frequencies of 20 MHz and above, ceramic piezoelectrics are thin and fragile, a problem not encountered with PVF.sub.2. In addition, PVF.sub.2 can function at higher power levels than many ceramic piezoelectric elements without suffering dielectric breakdown and subsequent failure. PVF.sub.2 films have also been used as diaphragms for loudspeakers and earphones. However, there is a drawback to making an effective electrical connection to PVF.sub.2 so that it can be used in such applications. One method that has been utilized in the attempt to overcome this problem has been to mechanically attach a lead wire via a spring clip to PVF.sub.2 film thinly coated with vacuum deposited aluminum. Another method has been to attach a wire lead to such an aluminized PVF.sub.2 film with electrically conductive silver-filled epoxy. However, such techniques suffer from various problems, including poor electrical interconnection, damage by mechanical contacts to film or aluminum coating, mechanical failure of lead attachment, and failure of adhesive to stick to aluminum metallization. Other problems encountered in use as an active device of such a transducer produced thereby include low current carrying capacity, burnout of the aluminum coating during normally anticipated service, arcing around points of contact and within such an adhesive conducting layer, anomalous beam patterns in the active transducer area near the lead attachment points, susceptibility of such contacts to corrosion, and bulkiness of such contacts. For the conductive adhesive connection, wherein the wire is lifted off the film, the poor dielectric strength of the adhesive and the presence of a high voltage across the epoxy can cause breakdown of the epoxy, thermal damage to the PVF.sub.2, and damage to the connection. Since PVF.sub.2 is temperature sensitive, and aluminum metallization applied thereto is thin, soldering a wire lead to aluminum metallization on a PVF.sub.2 film can cause permanent damage to the metallization and film.